sheila.dwyer@LIGO.ORG - posted 18:23, Monday 03 December 2018 - last comment - 20:49, Monday 03 December 2018(45665)
daytime work today, new ASC instabilities
Jenne, Hang, Sheila, Stefan
- This morning Jenne and Hang tried to use the alignment offsets set for 30Watts at 2W, and saw that the recycling gain was ~38 with those offsets at 2W.
- We then tried to revert the offsets to what we had before the vent, but saw the CSOFT instability when we started to increase the power.
- We have continued to have difficulties with the OMC alignment controls saturating. This is not likely to be due to the HAM6 HPI being locked, because it is only 10-20 urad away from it's unlocked position, and because the beam is centered on AS_A and AS_B using OMs.
- People were very enthusiastic about increasing the power, so we agreed to increase the power today and spend tomorrow after maintence on low noise locking at 20-25W.
- We have been loosing lock due to an instability which seems similar to what is described here: 43968 Jenne and Stefan widened the notches mentioned in that alog, but the problem is still occurring at slightly lower frequency.
- We have had instabilities when powering up due to the SRC pitch loops. This is new today, and is happening even when powering up to 20W, which had not been a problem before.
The instability that seems to grow most visible in CHARD actually can occur at two different frequencies: 21Hz and 28.8Hz.
The attached plot shows both frequencies going unstable at the same time. The peaks show up in the REFL DC centering loops and REFL9_Q. I could not find them on any OPLEV or whit ness channel.
We tried to lower the CHARD BW by more than x5, and put notches at the 28.8Hz (0.5Hz wide). None of that made any difference, suggesting that CHARD is just an observer of the problem.
We suspect that the MCL cross-over might have something to do with it. It has an incoherent feature in the transfer function at 29.3Hz. Note that the M2 notch was is off a bit (at 27.45 Hz). We should just disable it. Still that leaves the 21Hz to be explained.
(We once tried to turn off the notch and power up. We only saw the 21Hz instability that time.)
We we lowered the SRC2 PIT and YAW in the Guardian before the PWR-UP to 15 (from 100). Those gains eventually come down to 15 anyway. That seems to have solved the SRC runaway issue.